In a typical MRAM element, the relative orientations of magnetic directions of ferromagnetic layers define a binary state of the device. The resistance across an element is generally lowest when the magnetic directions of the ferromagnetic layers are in a parallel orientation and highest when the magnetic directions are in an antiparallel orientation.
One type of MRAM device is commonly referred to as a “spin valve.” GMR devices, including spin valves, can be used as data storage elements in magnetic random access memory (MRAM) devices. In this regard, exemplary MRAM applications of GMR devices are described in U.S. Pat. Nos. 6,147,922; 6,175,525; 6,178,111; 6,493,258, and U.S. Pat. App. Pub. No. 2005/0226064, all of which are incorporated herein by reference. Other types of MRAM devices include the magnetic tunnel junction (MJT), pseudo spin valve, and ring-shaped MRAM.
A spin valve typically includes two or more ferromagnetic layers that are separated by a thin layer of a non-magnetic metal (often copper) and also includes an antiferromagnetic layer that “pins” the magnetization direction of one of the ferromagnetic layers. FIG. 1 illustrates (in a simplified form) the layers in a typical spin valve 10 as seen from a side view. As shown in FIG. 1, spin valve 10 includes ferromagnetic layers 12 and 14 separated by a nonmagnetic layer 16. In a typical arrangement, one of the magnetic layers is configured to be a fixed layer 14. Fixed layer 14 is adjacent to an anti-ferromagnetic layer 18, such that the magnetization direction of fixed layer 14 is “pinned” in a particular orientation. The arrow in fixed layer 14 indicates an exemplary pinned orientation, though, in general, the orientation could be pinned in either direction. Thus, the magnetization direction of fixed layer 14 remains relatively fixed when operational magnetic fields are applied to spin valve 10. A second magnetic layer 12 is termed a free layer 12. In contrast with the fixed layer 14, the magnetization direction of free layer 12 is free to switch between parallel and antiparallel orientations, as indicated by the double-arrow symbol in free layer 12. By applying an appropriate magnetic field to spin valve 10, the magnetization direction of free layer 12 can be inverted while the magnetization direction of fixed layer 14 remains the same.
At least for MJT elements, the magnetic fields may applied by sending a current through copper interconnects that are cladded with a high permeability layer used to focus magnetic flux. Studies have shown that cladding increases the strength of the magnetic field in the direction of the element, and thus can be used to reduce the power required for programming the elements. This process is discussed in more detail in “Durlam M, A low power 1 Mbit MRAM based on 1T1MJT bit cell integrated with copper interconnects, published at VLSI Symposium 2002” all of which is incorporated herein by reference. For instance, in Durlam paper, shows that the value of a normalized magnetic field may more than double or approximately double.
One or more reference elements may included in an MRAM circuit and used in determining a logical state of the various operational elements. It is generally important to design the reference bit to be almost identical to the data bit yet with enough differences to maintain its fixed logical state for reliable and robust operation. Because cladding increases and focuses the applied magnetic field, cladding can also inadvertently cause a reference element to switch its logical state—leading to operational error.